An image forming apparatus, which is configured to form an image by an electro-photographic method, has a high-voltage power supply for applying a predetermined bias to respective sections. For example, when the high-voltage power supply applies a bias to a charging roller or a transfer roller, a bias voltage is controlled such that a current flowing in the charging roller or the transfer roller becomes constant.
A related art high-voltage power supply is configured to, when the bias voltage of the charging roller or the transfer roller rapidly decreases in a short time, determine that dew condensation occurs and impedance of the charging roller or the transfer roller decreases.